Determination of pH and pCO.sub.2 has a wide variety of clinical and analytical applications. Recently, it has been discovered that it is possible to determine pH and pCO.sub.2 of a sample using fluorescent intensity measurements. See, for example, Yuan and Walt, "pH-Dependent Fluorescence of Merocyanine-Eosin-Labeled Water-Soluble Polymers", Macromolecules 23, 4611-15 (1990). However, fluorescent intensity measurements are disadvantageous in that they can be inaccurate and/or imprecise in view of photo-bleaching, light scattering off the tissues in the case of in vivo clinical work and high-absorbance by the sample.
Also, although it is generally known that the fluorescent lifetime changes, as opposed to the intensity, of a fluorescent emission can be measured, there is no suggestion in the prior art of fluorescent indicator molecules which would exhibit suitable lifetime changes to allow for the measurement of pH and pCO.sub.2 using fluorescent lifetime measurements. Moreover, it is difficult to obtain the desired spectral properties, sensitivity to pH or pCO.sub.2, and the desired pK (dissociation constant) all within a single chromophore.